In recent years, an electron transport layer for transporting electrons has been becoming more important in a photoelectric element that converts light into electrical energy, such as a power generating device. The electron transport layer is required to have a high electron transport property. In addition, in the electron transport layer, it is also important that an interface at which charge separation occurs (hereinafter, referred to as reactive interface) is sufficiently wide.
Such electron transport layers have been hitherto formed of metals, organic semiconductors, inorganic semiconductors, conductive polymers, conductive carbon, and the like.
For example, an electron transport layer has been proposed which is formed of an organic substance for which electrons are carriers, such as a fullerene, a perylene derivative, a polyphenylenevinylene derivative, and pentacene. Thus, the electron transport capability of the electron transport layer has been improved, and the photoelectric conversion efficiency of the photoelectric element has been improved (see Non Patent Literature 1 for the fullerene, see Non Patent Literature 2 for the perylene derivative, see Non Patent Literature 3 for the polyphenylenevinylene derivative, and see Non Patent Literature 4 for pentacene).
In addition, it has been reported that in a molecular device-type solar cell, a structure in which an electron-donating molecule (donor) and an electron-accepting molecule (acceptor) are chemically bonded to each other is formed as a thin film on a substrate (see Non Patent Literature 5).
In the electron transport layer reported in each Non Patent Literature described above, both improvement of the electron transport performance and sufficient widening of the reactive interface are not achieved at the same time.
For example, in a photoelectric element that includes an organic-type electron transport layer formed of a fullerene or the like, recombination of charge easily occurs after charge separation. Thus, the conversion efficiency is not sufficient. In a photoelectric element that includes an inorganic-type electron transport layer formed of titanium oxide or the like, a sufficiently wide reactive interface is not formed, and an electron conductive potential which affects an open-circuit voltage is determined uniquely by the constituent elements of the electron transport layer. Thus, the conversion efficiency is not sufficient.
Furthermore, a fullerene or the like for forming an organic-type electron transport layer has poor stability as a material due to its low molecular weight, and also has relatively high solubility to a solvent. Therefore, when an electron transport layer is formed of a fullerene or the like, flexibility in device designing is decreased.